I agree that the Vandellas were there to provide vocal backup for Martha. she herself has stated that they were not a "group" like the Supremes or Temptations, with a shared artistic vision of what they wanted to pursue and convey. it's not to say that the Vandellas weren't talented or necessary. It's like in an orchestra - 99% of the time the primary melody is being played by something like the 1st Violins or the flutes or something. rarely is it the violas or the contrabassoon. Sure there are passage, or in some cases songs, where they carry it. But they provide the inner tones and melodies. Essential to the overall

So this is very similar to Mary Wilson's comment about how all three of the Supremes were part of the whole [[yes I'm mentioning the Sups, but only because mary has been quoted in many interviews discussing this). But they were also a group and [[supposedly) partnering together on style, concepts to record, direction, etc. The Vandellas mostly just followed Martha's direction. Frankly from what I've heard, that's how things were with the DRATS and also once Scherrie joined the Sups. mary pretty much ran the group as she wanted in the later 70s

one thing that's curious is how Motown approached the FEMALE backing vocals as being far more expendable than the male. or maybe I'm just not as familiar with the men's recordings. but no one really seemed to care if the backgrounds on Sup, Van or Marvelettes was by group members or not. so long as it was female voices. But did Smokey ever record the Miracles without the three guys? did the Temps do any without their members? perhaps some groups went after a bigger sound and had additional voices on top of theirs. But I find it interesting that Smokey never replaced Pete, Bobby or Roger but never thought twice about replacing Mary and Cindy or Kathryn and Ann/Gladys.